Techniques: Murder Beam
Face left (no diagonal), charge your beam, then while charging hit start and have Charge/Ice/Wave/Spazer equipped. Then move the cursor to Speed Booster, and press the left and fire buttons at the same time to equip all 5 beams at once. Then press start again and release the fire button. This will create the Murder Beam on screen, a glowing ball of energy that carries great destructive power.
Make sure you don't have Power Bombs equipped before firing.
Also, be sure to unequip either Spazer or Plasma before firing the Arm Cannon again.
Examples of using the Murder Beam include Boss Tricks: Murder Beam on Mother Brain.